This invention relates to novel precipitated calcium carbonate particles of varying size and shape, and to methods for their preparation involving the use of basic calcium carbonate as a precursor.
Calcium carbonate particles of varying sizes and shapes are useful as fillers or as reinforcing materials for rubber, paper, plastics, paints and other materials. These particles, may be roughly divided into two groups, to wit: natural calcium carbonate particles and precipitated calcium carbonate particles. Natural calcium carbonate is prepared by mechanically grinding limestone to particle sizes as small as 0.5 microns, although it becomes increasingly difficult to grind limestone below one micron. Moreover, ground limestone particles are usually very irregular in shape, and ordinarily exhibit a broad particle size distribution.
General methods for production of precipitated calcium carbonates are concerned with the slaking of lime (calcium oxide), temperature of carbonation, and rate of introduction of carbon dioxide into the slaked lime. By appropriately controlling the solution environment, either calcite, aragonite, or vaterite is produced. Depending on the conditions of the environment, calcite can have either a prismatic, scalenohedral or rhombohedral crystal habit. Aragonite is acicular and takes the form of either single or clustered needles. Vaterite is generally spherical but is unstable and converts readily to calcite. The crystal morphologies of the products are determined by the concentration of calcium and hydroxyl ion in the specific solution environment during the nucleation and growth of calcium carbonate.
Size control by varying slake and carbonation conditions is generally related closely to the resultant morphology and is usually limited to a relatively narrow range for each type of product.
Morphology and size are also significantly influenced by addition of various additives to slaked lime before carbonation. This is illustrated by U.S. Pat. No. 4,714,603 in which a synthesis for the preparation of spherical calcite particles is described wherein a dissolved polyphosphate is added to slaked lime prior to the introduction of carbon dioxide. Particle sizes of between 2.5 and 10.0 microns are disclosed with the size depending upon the quantity of polyphosphate added.
Other additives such as polybasic organic acids and polysaccharides have also been found to exert some size control when they are added to lime prior to carbonation, the effect being dependent on the source, age and purity of the lime used. However, the efforts heretofore employed to effect size control of precipitated calcium carbonate particles involve the introduction of additives or other controlling factors prior to, or in the initial stages of, the direct carbonation of lime.
Basic calcium carbonate was described by G. Schimmel in an article appearing in Naturwissenschaftem, 57, (38) 1970. It was determined that the formula for basic calcium carbonate was 2CaCO.sub.3.Ca(OH).sub.2.1.5H.sub.2 O. The preparation of plate-shaped basic calcium carbonate has been described by H. Yamada in the Journal of Pulp and Paper Technology Society (Japan) 44, pages 62-69 (1990). The method disclosed therein described the addition of reagents such as saccharose and glutamic acid to lime prior to carbonation whereby crystals of platy basic calcium carbonate, as well as spherical vaterites, cubic or spindle shaped calcite and calcium carbonate hexahydrate were formed. A method of making basic calcium carbonate is also disclosed in an article by H. Yamada in Gypsum and Lime, 196 (1985). The synthesis appearing therein comprises preparing a slurry of calcium hydroxide by hydrating calcium oxide at a temperature of from 10.degree. C. to 80.degree. C., and thereafter cooling the slurry to a temperature of 15.degree. C. to 20.degree. C. Thereafter carbon dioxide is introduced at a rate and under prescribed reaction parameters to produce basic calcium carbonate.